In electronic devices such as mobile phones, tablet computers and notebook computers, microphones are usually included. A sound transmission channel of a microphone on an electronic device is usually disposed separately. A small hole provided in a casing of the electronic device can be used as the sound transmission channel of the microphone, and the small hole exposed on the casing affects aesthetics of the whole appearance of the electronic device product. In addition, a process to produce the sound transmission channel separately in the casing of the electronic device can incur an extra manufacturing cost of the electronic device.